In high precision screen printers that use screens or stencils to print inks, solder pastes and other diverse media onto a substrate, such as a ceramic tape or a fiberglass circuit board, it is critical to align the stencil or screen with the substrate prior to the printing process so that the media is deposited onto the substrate in accurate alignment to datum features on the substrate. A screen printer incorporating a video probe for aligning a screen with a substrate is disclosed in re-issued U.S. Pat. No. 34,615 to Freeman, incorporated herein by reference. The disclosed screen printer includes a video probe that is inserted between a screen and a substrate to alternately view the screen and the substrate to provide alignment prior to printing.
Another prior art apparatus for aligning a screen with a substrate in a screen printer is disclosed in U.S. Pat. No. 5,060,063 to Freeman, incorporated herein by reference. The alignment system disclosed in U.S. Pat. No. 5,060,063 is shown in FIG. 1. This system includes a video probe 100 that is placed between a screen 104 and a substrate 102 to align the screen with the substrate. The video probe captures images of the substrate and the screen, and a computer control system is used to drive motor stages to align the screen to the substrate (or vice versa). The video probe 100 is then removed from between the screen and the substrate prior to printing, so that the screen may be disposed immediately over the substrate to allow for printing.
The video probe 100 has a housing 106 with a central viewing bore 108 having an optical axis 110 coaxial with a video camera 112. Housing 106 also has a viewing channel 114 having a central viewing axis 116 perpendicular to optical axis 110. An imaging beamsplitter 118 is centered at the intersection of the optical axis 110 and the viewing axis 116 so that the video camera may simultaneously view the object 102 and the device acting on the object 104. An object illuminating beamsplitter 124 is disposed in the viewing channel 114 between the viewing beamsplitter 118 and the substrate 102 to direct illumination toward the substrate along the viewing axis 116. Similarly, a device illuminating beamsplitter 126 is disposed in the viewing channel between the viewing beamsplitter and the screen 104 to direct illumination toward the screen along the viewing axis.
Separate fiber optic light sources 128 and 130 supply light to illuminating beamsplitters 124 and 126 respectively. Each fiber optic source terminates in a closed cavity 132, 134 and illuminates a light disperser 136, 138 adjacent to the respective illuminating beamsplitter 124, 126. Each disperser produces an even white tone across the adjacent face of the illuminating beamsplitter, which in turn produces a uniform illumination of the object or the device.
In the apparatus shown in FIG. 1, an object image travels along the viewing axis 116 through the beamsplitter 124 and is directed by the imaging beamsplitter 118 along the optical axis 110 to the viewing camera 112. Similarly, a device image travels along the viewing axis 116 through the beamsplitter 126 and is directed by the imaging beamsplitter along the optical axis to the viewing camera. Based on the object and device images received by the camera, either the object or the device can be moved to bring the object and the device into proper alignment for printing.
In typical prior art devices, a minimum of two fiducial marks on the screen and on the substrate are used to provide alignment. Thus, the video probe is moved to at least two locations, and is then removed from between the screen and the substrate, before printing begins.